This invention relates to a composite sheet useful as a disposable backing sheet of an adhesive label assembly and to a method of preparing the composite sheet. The present invention is also directed to an adhesive label assembly having an adhesive label sheet releasably adhered to the composite sheet serving as a disposable backing sheet.
Recently, adhesive labels have been used in a wide variety of fields in the forms of, for example, labels, seals, stickers, tags and emblems. An adhesive label assembly is composed of a disposable backing sheet, a label sheet provided on the backing sheet, and an adhesive layer interposed between the backing sheet and the label sheet such that the label sheet is peelable together with the adhesive layer from the backing sheet. The backing sheet generally includes a paper layer on which a release liner is provided. Glassine paper or plain paper such as wood-free paper, glazed paper or kraft paper is generally used as the paper layer.
Because of its good light permeability, glassine paper has been widely used as the backing sheet. Glassine paper, which is made of a intensively beaten chemical pulp and which is subjected to calendaring, has a Canadian standard freeness value of 300 ml or less and porosity of about 20 to 30%. As compared with plain paper such as wood-free paper, glazed paper or kraft paper, therefore, glassine paper has poor disintegratability and is ill-suited for recycle use.
To improve disintegratability of the glassine paper while retaining desired light permeability, a lot of proposals have been made. Japanese Laid-Open Patent Publication No. H07-109699 proposes the use of a glassine paper having a Cobb size value (30 seconds) of 35-65 g/m2 and made of a wood pulp having a Canadian standard freeness value of 130 to 240 ml. Japanese Laid-Open Patent Publications No. H09-031898 proposes the use of a treated paper obtained by passing a glassine paper made of a wood pulp having a Canadian standard freeness value of 130 to 240 ml through a nip between a pair of rolls at 10-50 kg/cm, followed by application of an aqueous solution of polyvinyl alcohol. Japanese Laid-Open Patent Publications No. H09-41286 proposes providing a glassine paper with an undercoat containing polyvinyl alcohol having a polymerization degree of 1,500-2,700 and a saponification degree of 80-90 mole %. Japanese Laid-Open Patent Publications No. H09-217299 discloses a backing sheet having a glassine paper made of a mixture of 90 to 60 parts by weight of a highly disintegrated pulp having a Canadian standard freeness value of 100 to 250 ml with 10 to 40 parts by weight of a lowly disintegrated pulp having a Canadian standard freeness value of at least 400 ml. Japanese Laid-Open Patent Publications No. H10-204795 suggests providing a glassine paper with an undercoat containing a water soluble polymer and a pigment having a particle size of 0.4 μm or less.
Plain paper has better disintegratability but has lower light permeability than glassine paper. An adhesive label assembly whose backing sheet has poor light permeability is ill-suited for use with printers provided with a light transmission-type label position sensor. To improve the light permeability, therefore, it is necessary to reduce the thickness of the backing sheet. This results in difficulty in cutting the label sheet on the backing sheet into a plurality of labels and in difficulty in removing cut labels therefrom.
To improve light permeability of the plain paper while retaining desired disintegratability, a lot of proposals have been also made. Thus, Japanese Laid-Open Patent Publication No. H08-144198 proposes the use of paper layer having formed thereon a barrier layer including at least one resin selected from polyacrylamide-acrylate copolymers, styrene-butadiene copolymers and copolymers of polyvinyl alcohol with a polystyrene-acrylate polymer. Japanese Laid-Open Patent Publication No. H11-1897 discloses a backing sheet including a machine glazed paper layer, a coating of a water-soluble polymer provided on the paper layer, and a release liner provided on the polymer coating. The polymer coating has a Cobb size value (60 seconds) of 10 to 40 g/,m2 and a Beck's smoothness of 300 to 1000 seconds. Japanese Laid-Open Patent Publication No. 2001-271295 proposes forming a transparency improving layer over a wood-free paper to obtain light transmittance of 40-50%. Japanese Laid-Open Patent Publication No. H07-229096 proposes applying coating of styrene-butadiene latex on both sides of a paper to improve machinability of punching and slitting.
The known backing sheets, however, are still not fully satisfactory with respect to light permeability, disintegratability and punching machinability.
Plain paper when used as a disposable backing sheet has an additional problem because of higher ink absorption as compared with glassine paper. Thus, when an opposite side of the plain paper backing sheet from the releasable label sheet is printed, the ink penetrates into the backing sheet. Therefore, in order to obtain a desired print image density, it is necessary to increase the amount of the ink. This deteriorates the fineness of the pint image.